Chemical looping combustion (CLC) consists in contacting in an enclosure at high temperature a gaseous, liquid and/or solid hydrocarbon feedstock with an oxygen-carrying metallic oxide type solid. The metallic oxide yields part of the oxygen it contains, which takes part in the combustion of the hydrocarbons.
After combustion, the fumes predominantly contain carbon oxides, water and possibly hydrogen. In fact, it is not necessary to contact the hydrocarbon feedstock with air and the fumes are then predominantly made up of combustion gases and possibly of a dilution gas used for transport and fluidization of the particles or a gas (water vapour for example) promoting solid fuel gasification.
It is thus possible to produce, after condensation, predominantly nitrogen-free fumes with high CO2 contents (generally above 90 vol. %, or even 98 vol. %) allowing to consider CO2 capture and storage. The metallic oxide that has taken part in the combustion is then carried to another reaction enclosure where it is contacted with air in order to be re-oxidized.
Implementing a chemical-looping combustion method requires large amounts of metallic oxides in order to burn all of the fuel. These metallic oxides are generally contained either in ore particles or in particles resulting from industrial treatments (residues from the iron and steel industry or from the mining industry, used catalysts from the chemical industry or refining). It is also possible to use synthetic materials such as, for example, alumina or silica-alumina supports on which metals that can be oxidized (nickel for example) have been deposited. The maximum oxygen capacity really available varies considerably from one oxide to another, generally ranging between 0.1 and 15%, often between 0.3 and 6 wt. %. Implementation under fluidized bed conditions is therefore particularly advantageous for conducting the combustion. Indeed, the finely divided oxide particles circulate more readily in the combustion and oxidation reaction enclosures, and between these enclosures, if the properties of a fluid are conferred on the particles.
Patent application FR-2,850,156 describes a chemical-looping combustion method wherein the fuel is crushed prior to being fed into the reduction reactor operating under circulating fluidized bed conditions. The reduced size of the solid fuel particles allows more complete and faster combustion. Separation downstream from the circulating bed is first provided by a cyclone, then by a device allowing the unburnt particles to be separated from the metallic oxide particles. Unburnt particles entrainment in the oxidation zone and therefore CO2 emissions in the oxidation reactor effluents is thus prevented.
The separation device is fluidized by water vapour, which allows to separate the fine and light particles such as carbon-containing residues and to feed them into the reactor again, whereas the denser and bigger oxide particles are transferred to the oxidation reactor.
The object of patent application FR-2,896,709 is a particle separator operating in a chemical looping process. This separator is fed with the stream of oxygen-carrying particles mixed with the solid fuel. In the separator, the particles flow in the dense phase by following a tortuous path and passing through baffles, which allows to control the residence time and to promote separation of the lighter particles (unburnt particles) from the heavier particles (metallic oxides). The particles are then fed into a fluidization zone, said fluidization being controlled by means arranged at the base of the separator as well as on the deflector walls, which allows the lighter particles to be entrained and recycled to the fluidization reactor.
The operation of the separator according to document FR-2,896,709 however induces several not insignificant drawbacks.
Indeed, the separation efficiency is reduced due to a limitation of the particle entrainment capacity in the gas phase. In fact, for large amounts of particles to be carried along, large amounts of gas and free sections are required.
Furthermore, in the dense phase, it is impossible to totally separate two different solid phases. In fact, the ascending motion of the gas bubbles paradoxically causes the lighter particles to descend again towards the dense phase and prevents total separation.
Besides, the geometry of the separator of document FR-2,896,709 is very complicated due to the presence of internals, notably asymmetrical internals. This geometry is problematic considering the mechanical stresses undergone by the materials that make up the separator under the temperature conditions of the CLC process, generally above 800° C. Indeed, although the external walls are often protected by refractory cement and remain at low temperature, this is not the case for the internal walls that are subjected to the process conditions.
The applicants have developed a new separator wherein the particle mixture coming from the combustion zone is contacted with a gaseous stream coming from the combustion zone and/or from an external gas source. This mixture then enters the dilute phase of the separator. In the separator, the velocity of the gas flow is controlled so as to allow the heavier particles to settle (essentially metallic oxide particles), the lighter particles (essentially unburnt particles) being carried along to the upper part of the separator in order to be recycled to the combustion zone.
The separator according to the invention thus has improved unburnt particles and metallic oxide particles separation efficiency.
Furthermore, the relatively simple design of the separator allows the problems linked with thermal stresses to be overcome.